'\" te
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.TH ELF_UPDATE 3ELF "Jul 11, 2001"
.SH NAME
elf_update \- update an ELF descriptor
.SH SYNOPSIS
.LP
.nf
cc [ \fIflag\fR ... ] \fIfile\fR ... \fB-lelf\fR [ \fIlibrary\fR ... ]
#include <libelf.h>

\fBoff_t\fR \fBelf_update\fR(\fBElf *\fR\fIelf\fR, \fBElf_Cmd\fR \fIcmd\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBelf_update()\fR function causes the library to examine the information
associated with an \fBELF\fR descriptor, \fIelf\fR, and to recalculate the
structural data needed to generate the file's image.
.sp
.LP
The \fIcmd\fR argument can have the following values:
.sp
.ne 2
.na
\fB\fBELF_C_NULL\fR\fR
.ad
.RS 15n
This value tells \fBelf_update()\fR to recalculate various values, updating
only the \fBELF\fR descriptor's memory structures. Any modified structures are
flagged with the \fBELF_F_DIRTY\fR bit. A program thus can update the
structural information and then reexamine them without changing the file
associated with the \fBELF\fR descriptor. Because this does not change the
file, the \fBELF\fR descriptor may allow reading, writing, or both reading and
writing (see  \fBelf_begin\fR(3ELF)).
.RE

.sp
.ne 2
.na
\fB\fBELF_C_WRITE\fR\fR
.ad
.RS 15n
If \fIcmd\fR has this value, \fBelf_update()\fR duplicates its \fBELF_C_NULL\fR
actions and also writes any ``dirty'' information associated with the \fBELF\fR
descriptor to the file. That is, when a program has used
\fBelf_getdata\fR(3ELF) or the \fBelf_flagdata\fR(3ELF) facilities to supply
new (or update existing) information for an \fBELF\fR descriptor, those data
will be examined, coordinated, translated if necessary (see
\fBelf32_xlatetof\fR(3ELF)), and written to the file. When portions of the file
are written, any \fBELF_F_DIRTY\fR bits are reset, indicating those items no
longer need to be written to the file (see \fBelf_flagdata\fR(3ELF)). The
sections' data are written in the order of their section header entries, and
the section header table is written to the end of the file. When the \fBELF\fR
descriptor was created with \fBelf_begin()\fR, it must have allowed writing the
file. That is, the \fBelf_begin()\fR command must have been either
\fBELF_C_RDWR\fR or \fBELF_C_WRITE\fR.
.RE

.sp
.LP
If \fBelf_update()\fR succeeds, it returns the total size of the file image
(not the memory image), in bytes. Otherwise an error occurred, and the function
returns \fB\(mi1\fR\&.
.sp
.LP
When updating the internal structures, \fBelf_update()\fR sets some members
itself. Members listed below are the application's responsibility and retain
the values given by the program.
.sp
.LP
The following table shows ELF Header members:
.sp

.sp
.TS
l l
l l .
Member	Notes
	
e_ident[EI_DATA]	Library controls other \fBe_ident\fR values
e_type	
e_machine	
e_version	
e_entry	
e_phoff	Only when \fBELF_F_LAYOUT\fR asserted
e_shoff	Only when \fBELF_F_LAYOUT\fR asserted
e_flags	
e_shstrndx	
.TE

.sp
.LP
The following table shows the Program Header members:
.sp

.sp
.TS
l l
l l .
Member	Notes
	
p_type	The application controls all
p_offset	program header entries
p_vaddr	
p_paddr	
p_filesz	
p_memsz	
p_flags	
p_align	
.TE

.sp
.LP
The following table shows the Section Header members:
.sp

.sp
.TS
l l
l l .
Member	Notes
	
sh_name	
sh_type	
sh_flags	
sh_addr	
sh_offset	Only when \fBELF_F_LAYOUT\fR asserted
sh_size	Only when \fBELF_F_LAYOUT\fR asserted
sh_link	
sh_info	
sh_addralign	Only when \fBELF_F_LAYOUT\fR asserted
sh_entsize	
.TE

.sp
.LP
The following table shows the Data Descriptor members:
.sp

.sp
.TS
l l
l l .
Member	Notes
	
d_buf	
d_type	
d_size	
d_off	Only when \fBELF_F_LAYOUT\fR asserted
d_align	
d_version	
.TE

.sp
.LP
Note that the program is responsible for two particularly important members
(among others) in the \fBELF\fR header. The \fBe_version\fR member controls the
version of data structures written to the file. If the version is
\fBEV_NONE\fR, the library uses its own internal version. The
\fBe_ident[EI_DATA]\fR entry controls the data encoding used in the file. As a
special case, the value may be \fBELFDATANONE\fR to request the native data
encoding for the host machine. An error occurs in this case if the native
encoding doesn't match a file encoding known by the library.
.sp
.LP
Further note that the program is responsible for the \fBsh_entsize\fR section
header member. Although the library sets it for sections with known types, it
cannot reliably know the correct value for all sections. Consequently, the
library relies on the program to provide the values for unknown section types.
If the entry size is unknown or not applicable, the value should be set to
\fB0\fR.
.sp
.LP
When deciding how to build the output file, \fBelf_update()\fR obeys the
alignments of individual data buffers to create output sections. A section's
most strictly aligned data buffer controls the section's alignment. The library
also inserts padding between buffers, as necessary, to ensure the proper
alignment of each buffer.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Stable
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
.BR elf (3ELF),
.BR elf32_fsize (3ELF),
.BR elf32_getehdr (3ELF),
.BR elf32_getshdr (3ELF),
.BR elf32_xlatetof (3ELF),
.BR elf_begin (3ELF),
.BR elf_flagdata (3ELF),
.BR elf_getdata (3ELF),
.BR libelf (3LIB),
.BR attributes (7)
.SH NOTES
.sp
.LP
As mentioned above, the \fBELF_C_WRITE\fR command translates data as necessary,
before writing them to the file. This translation is \fInot\fR always
transparent to the application program. If a program has obtained pointers to
data associated with a file (for example, see \fBelf32_getehdr\fR(3ELF) and
\fBelf_getdata\fR(3ELF)), the program should reestablish the pointers after
calling \fBelf_update()\fR.
